This invention relates to conduits for transporting non-volatile liquids (such as oils or lubricants) in which the conduit""s internal surfaces have a surface energy lower than that of the nonvolatile liquid so as to provide for enhanced transportability of the liquid due to de-wetting of the surface by the liquid and lack of build-up of the liquid on the surface. This invention has applicability to any internal tube, pipe or channel surface where a non-volatile liquid has to be transported by a vapor, gas, liquid or two phase vapor/liquid mixture. One such example is the transport of gas/oil mixtures through pipelines. Another example is where the conduit is in the form of a heat exchanger, especially where the heat exchanger is part of a refrigeration system. Thus, in a refrigeration system having a compressor, an evaporator heat exchanger, a condenser heat exchanger, and liquid and vapor lines, where the non-volatile liquid is a lubricant and where a refrigerant is also transported through the system, transportability of the lubricant can be enhanced by at least providing the internal surfaces of the evaporator heat exchanger with a surface energy lower than that of the lubricant. The use of this invention in such systems not only enhances the transportability of the lubricant (thus enhancing lubricant return to the compressor and reducing oil retention in the heat exchanger) but also improves the system""s performance in terms of refrigeration capacity and coefficient of performance (xe2x80x9cCOPxe2x80x9d), thus enabling the use of smaller evaporators for a specified cooling load. As used herein xe2x80x9crefrigeration systemsxe2x80x9d include air conditioning systems.
In current refrigeration systems, a small fraction of the lubricant from the compressor is carried over and circulated through the rest of the system. Some amount of this lubricant is usually retained in the heat exchanger(s), forming a thin layer which inhibits heat transfer. Thus, excessive retention of lubricant adversely affects system performance. To minimize lubricant retention and separating out of the lubricant in the heat exchangers and connecting lines, the lubricant had to be fully miscible with the refrigerant. Further, hydrofluorocarbon (HFC) refrigerants such as 1,1,1,2-tetrafluoroethane (134a) require use of miscible polyol ester (POE) lubricants since conventional mineral oil (MO) or alkyl benzene (AB) lubricants are not miscible with HFCs. Apart from being more expensive than MO or AB lubricants, POE lubricants also require a much cleaner compressor manufacturing facility since they are hygroscopic in nature. Thus, it would be useful to find a way to enable the use of immiscible lubricants with HFC refrigerants and to enhance return of lubricant, regardless of its nature, to the compressor.
A method for enhancing the transportability of a non-volatile liquid in a conduit (such as in the form of a heat exchanger) is provided, which method comprises providing the internal surfaces of the conduit with a surface energy lower than that of the non-volatile liquid. A preferred embodiment involves a method for enhancing the transportability of a lubricant in a refrigeration system, the refrigeration system having a compressor, an evaporator heat exchanger, a condenser heat exchanger, and liquid and vapor lines, the lubricant and a refrigerant being transported through the system, the method comprising providing the internal surfaces of the evaporator heat exchanger with a surface energy lower than that of the lubricant. Although not essential, the internal surfaces of the condenser heat exchanger and the liquid and vapor lines can also be provided with a surface energy lower than that of the lubricant. In one embodiment, the internal surfaces of the system""s heat exchangers and, optionally, other system components will have a surface energy greater than that of the refrigerant to enable wetting of the surface by the refrigerant, making it easier for the refrigerant flow to push the lubricant along the surface of the heat exchangers. In another embodiment, however, the condenser heat exchanger can be provided with a surface energy which is lower than that of the refrigerant to promote drop-wise condensation and thus enhance heat transfer.
In refrigeration systems, the non-volatile liquid(s) include conventionally used lubricants, such as MO, AB, POE, polyalkyl glycol, and polyvinyl ether, as well as additives used to enhance system performance, such as tetraglyme. The refrigerants include fluorocarbons, ammonia, carbon dioxide, and hydrocarbons. A typical refrigerant is 134a.
It has now been found that the transportability of a non-volative liquid in a conduit can be improved by providing the internal surfaces of the conduit with a surface energy lower than that of the non-volatile liquid.
The desired surface energy can be achieved by any of several known methods for altering the surface energies of solids. Examples are chemical surface modification, such as direct fluorination of a metal surface, or application of a thin organic or additive-containing composite coating. An example of a composite coating is Ni-flor, a nickel-phosphorous matrix containing polytetrafluoroethylene particles which is available from Atotech Inc. Organic coatings include polymers such as polyethylene, polypropylene, polystyrene, polymethyl methacrylate, polyethylene terephthalate, nylon 6, polydimethylsiloxane, polycarbonate of bisphenol-A, polyheptafluoroisopropyl acrylate, polytetrafluoroethylene, polyvinyl fluoride, polychlorotrifluoroethylene and polyvinylidene fluoride, the latter polymer having been found to be particularly useful for refrigerant applications where the surface is commonly copper, aluminum or steel. Polyvinylidene fluoride (xe2x80x9cPVDFxe2x80x9d) as used herein refers not only to the homopolymer of vinylidene fluoride (xe2x80x9cVDFxe2x80x9d) but also to copolymers prepared from at least about 85 weight % VDF monomer and up to about 15 weight a hexafluoropropylene (HFP). Examples of such polymers include Kynar(copyright) 741 (polyvinylidene fluoride), Kynar Flex(copyright) 2801 (a VDF/HFP copolymer containing about 10% HFP) and Kynar Flex 2751 (a VDF/HFP copolymer containing about 14% HFP), available commercially from Elf Atochem North America, Inc. of Philadelphia, Pa. Some HFP (up to about 15% by weight) is useful in the PVDF because having HFP present in the monomer blend makes the coating easier to solution cast and contributes to flexibility and elasticity of the polymer, thereby enabling the coating to adhere to the internal surfaces as they elongate or contract during temperature cycling. Since HFP also has a lower surface energy (about 16 dyn/cm) than VDF, it can also be used to customize the polymer""s surface energy. On the other hand, more than 15% HFP is preferably avoided in order to minimize mass gain through contact with the refrigerant and lubricant.
Surface energies for the foregoing types of organic polymers can be found in the Table of Surface Energies for Common Polymers in the Polymer Handbook: 3rd Edition, Wiley, 1989. For example, the preferred polyvinylidene polymers typically have a surface energy at 20xc2x0 C. in the range of 25-32 dyn/cm (dynes/centimeter), while a refrigerant such as 134a has a surface energy of about 1.5 to 19 dyn/cm over a temperature range of from about 80xc2x0 C. to about xe2x88x9250xc2x0 C., a typical MO used in refrigeration applications has a surface energy of about 47 dyn/cm at room temperature, and AB oils have a typical surface energy in the range of 35 to 45 dyn/cm at room temperature. Accordingly, use of a PVDF coating in a refrigeration system application will inhibit wetting of the interior surfaces by the lubricant but permit wetting by the refrigerant. Tests done on the Ni-flor composite show it also has a surface energy in the desired range, about 15-30 dyn/cm.
A relatively thin coating (desirably no more than about 2 microns) is preferred in order to minimize altering the system""s thermal performance (heat transfer) and to improve adhesion. Methods of applying coatings to metal surfaces are well-known, such as spray, dip or curtain coating.
The practice of the invention is illustrated in more detail in the following non-limiting examples.